The technology for the production of aluminum by the electrolysis of alumina, dissolved in molten cryolite containing sales, at temperatures around 950.degree. C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Heroult, has not evolved as many other electrochemical processes. It is difficult to understand why, despite the tremendous growth in the total production on aluminum that in fifty years has increased almost one hundred fold, the process and the cell design have not undergone any great change or improvement.
The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by anthracite-based carbon blocks at the wall and at the cell floor bottom which acts as cathode and which the negative pole of a direct current source is connected by means of steel conductor bars embedded in the carbon blocks.
The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still approximately 950.degree. C. in order to have a sufficiently high alumina solubility and rate of dissolution which decreases rapidly at lower temperatures.
The carbonaceous materials used in Hall-Heroult cells as the anode and as the cell lining are certainly not ideal for resistance under the existing adverse operating conditions.
The anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form CO.sub.2 and small amounts of CO. The actual consumption of the anode is approximately 450 Kg/Ton of aluminum produced which is more than 1/3 higher than the theoretical amount of 335 Kg/Ton corresponding to that of the stoichiometric reaction.
The carbon lining of the cathode bottom has a useful life of a few years after which the operation of the entire cell must be shopped and the cell relined at great cost. In spine of an aluminum pool having a thickness of more than 20 mm maintained over the cathode, the deterioration of the cathode carbon blocks cannot be avoided because of penetration of cryolite and liquid aluminum, as well as intercalation of sodium ions which causes swelling and deformation of the cathode carbon blocks and displacement of such blocks.
In addition, when cells are rebuilt, there are problems of disposal of the carbon which contains toxic compounds including cyanides.
The carbon blocks of the cell wall lining do not resist attack by cryolite, and a layer of solidified cryolite has to be maintained on the cell wall to extend its life.
The major drawback, however, is due to the fact that irregular electromagnetic forces create waves in the molten aluminum pool and the anode-cathode distance (ACD), also called interelectrode gap (IEG), must be kept at a safe minimum value of approximately 50 mm to avoid short circuiting between the cathodic aluminum and the anode.
The high electrical resistivity of the electrolyte, which is about 0.4 Ohm.cm., causes a voltage drop which alone represents more than 40% of the total voltage drop with a resulting energy efficiency which reaches only 25% in the most modern cells.
The high increase of the cost of energy, which has become even a bigger item in the total manufacturing cost of aluminum since the oil crisis, has decreased the rate of growth of this important metal.
In the second largest electrochemical industry following aluminum, namely the chlorine and caustic industry, the invention of dimensionally stable anodes (DSA.RTM.) which were developed around 1970 permitted a revolutionary progress in chlorine cell technology resulting in a substantial increase in cell energy efficiency, in cell life and in chlorine caustic purity.
The substitution of graphite anodes with DSA.RTM. increased drastically the life of the anodes and reduced substantially the cost of operating the cells. The rapid increase in chlorine caustic growth was stopped only by ecological concerns.
In the case of aluminum production, pollution is not due to the aluminum produced, but to the materials used in the process and to the primitive cell design and operation which have remained the same over the years.
Progress has been made in the operation of modern plants which utilize cells where the gases emanating from the cells are in large part collected and adequately scrubbed and where the emission of highly polluting gases during the manufacture of the carbon anodes is carefully controlled.
However, the frequent substitution of the anodes in the cells is still a clumsy, unpleasant and expensive operation. This cannot be avoided or greatly improved due to the size and weight of the anode and the fact that the cathode is formed by the cell floor and is not removable during cell operation. Recently, progress has been made in the anode and the cathode composition, primarily with the development of non-carbon, substantially non-consumable anodes (NCA) and cathodes (NCC). The life of these NCA and NCC is nevertheless limited and even these electrodes need occasional replacement or reconditioning.